1. Field of the Invention
The present invention relates generally to dynamoelectric machines, and, more particularly, is concerned with a coil carrier fixture for mounting a field coil to a rotor of a dynamoelectric machine, such as an AC and DC motor-generator set, and a method of fabricating a field coil carrier assembly.
2. Description of the Prior Art
A secondary power source in the form of a dynamoelectric machine is typically provided in a nuclear submarine to generate reserve power. A nuclear power plant provided in the submarine is the primary power source to power the submarine. It also powers the secondary power source to charge DC batteries. Then, during periods when the nuclear power plant is shut down or knocked off-line, the reserve power of the DC batteries is used to restart the primary source to operate the submarine.
One typical secondary power source used heretofore employs a dynamoelectric machine in the form of an AC and DC motor-generator set in which the AC and DC components are disposed in tandem arrangement on and along a common solid shaft. During periods of nuclear power plant operation, the AC component functions as a motor to rotatably drive the common shaft and thereby run the DC component as a generator to charge the DC batteries. Then, during periods when the nuclear power plant is shut down, the DC batteries operate the DC component as a motor to rotatably drive the common shaft and thereby run the AC component as a generator to operate the submarine.
The AC and DC motor-generator set typically includes AC and DC rotors supported in tandem relationship along a central shaft for rotation therewith. The AC and DC rotors include respective pole body spiders and pole heads fastened on circumferentially spaced outer projections of the pole body spiders. Also, field coils are installed about the spaced projections of the rotor pole body spiders.
The conventional method for mounting a field coil to the rotor is to first wind each coil on a mandrel, next varnish the coil, and then wrap the entire coil with insulation tape. The insulated coils are then slipped onto the projections of the pole body spider of the rotor. Finally, the entire rotor (with coils mounted) is varnished by dipping to ensure a firm fixed fit of the coils to the rotor pole body. The inherent problem with this mounting method is the difficulty of removing and servicing an individual coil once it is varnish glued to the rotor. Also, once the coil is replaced the entire rotor has to be lifted and dipped in varnish to secure the newly added coil.
An alternate method for mounting field coils to the rotor is to wind the individual coils directly on the projections of the rotor pole body spider or on a metal one-piece annular bobbin. This method requires great care to ensure a proper insulation layer on the outer geometry of the pole body/bobbin winding surface to prevent electrical shorting of the coil.
Resin-based, as opposed to metal, bobbins exist which provide an insulated winding surface without the difficulty of having to develop and ensure the integrity of an applied insulation process. However, fabrication of a resin-based bobbin requires relatively expensive mold tooling which is: considered excessive for only low production quantities.
In addition, using a bobbin to mount the field coil to the rotor pole body is tedious and time-consuming. It generally requires extensive custom shimming of the bobbin to provide a satisfactory tight fit of the bobbin to the rotor that is long-lasting and will eliminate the possibility of the field coil rocking during machine operation. Furthermore, a field coil which is tightly shimmed to the rotor to prevent rocking will ordinarily result in a mounted field coil which is difficult to remove for retrofit.
Consequently, a need still exists for another approach to mounting individual field coils to the rotors of a dynamoelectric machine, such as an AC and DC motorgenerator set, so as to avoid the above-described drawbacks of the prior methods.